1. Field of the Invention
The present invention relates to a device for controlling an aerodynamic surface for balancing a helicopter in terms of pitch.
2. Description of the Related Art
It is known that in hovering flight, balancing the pitching moment of the helicopter with respect to a fixed point involves two main components: the moment due to the weight of the aircraft and the moment due to the main lift and propulsion rotor which, for a given mass, is proportional to the tilting of the rotor with respect to the plane perpendicular to the rotor mast. Thus, variations in the center of gravity of the helicopter give rise to variations in the balanced attitude.
It is also known that when the helicopter is in cruising flight, another component of the pitching moment comes into play, namely the aerodynamic moment exerted on the cell. In the case of a fuselage, this component has a destabilizing effect, that is to say that for a variation in incidence there is produced an aerodynamic pitching moment on the fuselage which tends to move it away from its position of equilibrium. The consequence of this unstable component is that of increasing the variations in attitude which are associated with the center of gravity compared with those which exist in hovering flight.
These variations in attitude have negative consequences. Excessively nose-down attitudes considerably increase the drag of the helicopter and therefore reduce its maximum speed. They also lead to a feeling of discomfort for the crew and passengers. Excessively nose-up attitudes lead to high moments on the mast and hub of the main rotor, which affects the life of these components.
To improve the balance of the helicopter in terms of pitching moment, it is known practice to have an aerodynamic surface for pitch attitude control placed toward the rear of the aircraft and which compensates for the instability of the pitching moment on the fuselage.
In consequence, to obtain a balancing empennage which is efficient, its configuration needs to be defined in such a way as to master the compromise between the two aforementioned constraints (performance/hub loading) which are, moreover, contradictory. In effect, to optimize the performance at high speed, said aerodynamic surface needs to be formed in such a way that it yields an attitude close to zero attitude, and this is penalizing on the moments at the center of the hub, where it is desirable to have a very nose-down attitude in order to reduce the fore-and-aft flapping.
In addition, the compromise obtained needs to be satisfactory for various mass, altitude and center of gravity scenarios and possibly for the various possible external aerodynamic configurations.
One solution, in an attempt to satisfy the aforementioned conditions, is to increase the efficiency of said empennage, that is to say increase its area, so as to reduce the effects of the disturbances that the variations in mass and center of gravity constitute.
However, such a solution is limited, particularly by certain constraints associated with the area of the empennage, namely, in particular:
a constraint known as the "attitude hump", which is due to the interactions between the rotor and the empennage at low speeds and which results in a nose-up effect which can reduce visibility during an approach. To solve this problem, either the area of said empennage has to be restricted or this empennage has to be turned, if possible, so as to reduce its angle of incidence during these interactions; PA1 a constraint associated with the coupling when climbing/diving, which depends directly on the area of the empennage, whose angle of incidence in oblique flight causes a substantial pitching moment variation and results in substantial variations in the attitude and in the position of the fore-and-aft cyclic stick; and PA1 constraints of size and in the folding of the tail boom in the case of a helicopter on board a carrier, for example. PA1 in a first type of solution, said control means are built into said pitch-control channel and control said aerodynamic surface and said main rotor simultaneously. In consequence, the device in accordance with the invention in this case is alone in performing two controls; and PA1 in a second type of solution, said control means are added to said pitch-control channel and control only said aerodynamic surface, which avoids modifying the pitch-control channel. This last type of solution can therefore be applied to any type of helicopter because it requires no modifications to said control channel. PA1 means for adjusting said neutral point, as a function of characteristic parameters; and/or PA1 adjustable stops limiting the movement of the mechanical connection between the first branch of the bell crank and the member for actuating the aerodynamic surface; and/or PA1 means for adjusting said stops as a function of characteristic parameters; and/or PA1 a damper connected to the mechanical connection between the first branch of the bell crank and the member for actuating the aerodynamic surface. PA1 a conversion between said control commands for, respectively, said aerodynamic surface and said main rotor, using a pitching moment efficiency conversion gain; and/or PA1 low-pass filtering of said second control command. PA1 a calculation unit for determining a speed control command for the aerodynamic surface from the difference between the pitch-control command and the reference command; and PA1 means for actuating said aerodynamic surface in terms of speed as a function of the control command thus determined.